Weep so bitterly at first (3)

I believe the answer is:

sob

'weep' is the definition.
(I know that sob is a more specific form of the action the weep)

'so bitterly at first' is the wordplay.
'at first' suggests taking the first letters.
The initial letter of 'bitterly' is 'b'.
'so'+'b'='SOB'

(Other definitions for sob that I've seen before include "Cry uncontrollably" , "Cry convulsively" , "making one gasp" , "Weep heavily, wail" , "Shake with tears" .)

I've seen this clue in the Evening Standard.

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